This invention relates to a method for making a printing plate, which comprises electrophotographically forming a toner image by using an organic photoconductive compound and subjecting non-image areas other than the toner image areas to an etching treatment with a solution containing an alkali and/or an alcohol as major component.
Various printing plates employing organic photoconductive compounds have been known. As examples, mention may be made of those disclosed by Japanese Patent Publication Nos. 17,162/62, 6,961/63, 2,426/66, and 39,405/71; Japanese Patent Application "Kokai" (Laid-open) Nos. 19,509/75, 19,510/75, 145,538/79, 89,801/79, 134,632/79, 19,803/79, and 105,244/80. The binders used in these printing plates include styrene-maleic anhydride copolymers, maleic ester resins, vinyl acetate-crotonic acid copolymers, vinyl acetate-maleic anhydride copolymers, and phenolic resins.
The printing plates employing these conventional resins as the binder for a photoconductive compound, however, exhibit various disadvantages, especially when a high printing endurance is required. If a styrene-maleic anhydride copolymer is used as the binder, the coating film which is formed is hard and subject to cracking when the printing plate is bent. This seems to be caused by the relatively rigid and crystalline nature of the polymer molecule, as is evidenced by the low peel resistance in the tape test for adhesion measurement as described later. Although it is proposed to incorporate a plasticizer to improve properties of the resin, yet the plasticizer tends to deteriorate the electrostatic characteristics. If a high-boiling oil is used as the plasticizer, it is apt to migrate to the surface, thus decreasing the storage stability. The use of phenolic resins as binders presents problems on physical properties of the printing plate, similarly to the styrene-maleic anhydride copolymer. Phenolic resins are generally low in molecular weight and solution viscosity. Moreover, they form brittle films. A vinyl acetate-crotonic acid copolymer or a vinyl acetate-maleic anhydride copolymer may be called a fairly suitable binder for use in the printing plate. However, it has a disadvantage of causing the blinding of details of an image, if the number of copies reaches several tens of thousands. When an acrylic resin, a homopolymer of acrylic or methacrylic esters, is used as the binder, although an image can be formed by electrophotography, yet the etching treatment with an aqueous alkaline or alcoholic solution is not so easy, thus making it difficult to achieve the object of this invention. A maleic acid resin formed by the reaction of rosin and maleic anhydride is generally of low molecular weight; an ester of this resin is also unsuitable for the printing plate aimed at in this invention.
As a result of an extensive study to overcome the above difficulties of conventional binders, it was found that the object of this invention can be achieved by using as the binder a half-ester resin of a styrene-maleic anhydride copolymer.